Light from the two images takes different routes to Earth. When the quasar’s brightness fluctuates, the two images flicker one after another, rather than at the same time. The delay in time between those two flickers, along with information about the meddling galaxy’s gravitational field, can be used to trace the light’s journey and deduce the distances from Earth to both the quasar and the foreground galaxy. Knowing the redshifts of the quasar and galaxy enabled the scientists to estimate how quickly the universe is expanding.
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Clever. I wish I would have thought of it.
Lensing galaxies are a relatively new discovery. The newspapers reported astronomers finding McDonald’s arches in the galaxy, and couldn’t figure it out for a while.